Method for producing optical disc substrates

ABSTRACT

Provided is a method for producing optical disc substrates having a low degree of birefringence. The method has good mass-producibility, in which the pattern transferability on the substrates produced is good. In producing optical disc substrates having a diameter of from 80 to 120 mm and a thickness of from 0.5 to 0.7 mm, through injection molding or injection-compression molding, a resin for the substrates is injected and charged into the cavity of a mold at a resin filling rate of not lower than 65 cm 3 /sec, said resin filling rate being obtained by dividing the cavity volume (cm 3 ) of the mold, into which the resin is charged, by the time (sec) taken from the start of resin injection through the tip of the nozzle of the injection-molding machine to the arrival of the resin at the deepest end of the cavity of the mold.

FIELD OF THE INVENTION

The present invention relates to a method for producing optical discsubstrates.

BACKGROUND OF THE INVENTION

Optical discs which have heretofore been developed include read-onlydiscs such as compact discs (CD), laser discs (LD), etc.; rewritablediscs such as magneto-optical discs (MO discs), etc.; and write oncediscs such as recordable CD (CD-R), etc. FIG. 3 is a perspective viewschematically showing one example of such conventional optical discs. Asin FIG. 3, in general, an optical disc comprises a substrate with guidegrooves (or pits) 1, 2, each having a track pitch P of a predeterminedsize and a predetermined depth D. For reading or rewriting theinformation recorded in the optical disc, a laser ray having beenfocused through a lens to have a wavelength of about 800 nm isirradiated to the pits or to the recorded information on the guidegrooves. The information capacity of conventional optical discs is about640 MB/disc.

Recent developments in the multimedia industry are noticeable. Withthose, desired are high-density optical discs which can recordlarge-capacity information such as image information and are compact.However, conventional CD and MO discs could not meet the requirements inthe market, as their memory capacity is insufficient.

Given that situation, high-density read-only optical discs (so-calledDVD) having a diameter of 120 mm, which is the same as that of CD, buthaving a recording density (4.7 GB) of at least 7 times that of CD arebeing developed (for example, see “Industrial Materials”, Vol. 44, No.10, pp. 103-105, 1996). DVD comprises a laminate of two substrates, eachof which is thinner than the substrate of conventional optical discs.The laser ray to be used for reading the information recorded on DVD hasa wavelength of 650 nm or 635 nm. Therefore, the wavelength of the laserray to be used for reading the information recorded on DVD is shorterthan that (about 800 nm) of the laser ray for reading CD. The wavelengthof the laser ray to be used for reading the recorded information is inproportion to the spot diameter of the layer ray as focused through alens. Using a laser ray having a shorter wavelength makes it possible torecord and reproduce (or that is, to write and read) higher-densityinformation, as the laser ray shall have a smaller spot diameter. Thetrack pitch P of DVD is 0.74 μm, which is about ½ of the track pitch(1.6 μm) of conventional CD. Therefore, the recording density of DVD isgreatly increased.

On the other hand, high-density rewritable optical discs (so-calledDVD-RAM), of which the guide grooves formed on the substrate have atrack pitch P of 1.48 μm and a groove width of 0.74 pm. For informationrecording on those optical discs, a laser ray having the same wavelengthas that for DVD noted above is irradiated to both the inside of theguide grooves (hollows) and the top between the adjacent guide grooves(hills), to thereby accomplish information writing and rewriting throughphase conversion (see, for example, Asakura's “DVD”, pp. 126-134,published by Ohm Co., 1996). DVD-RAM of that type has the same outerdiameter (120 mm) as CD, but has a recording density (2.6 GB on onesurface) of about 4 times that of CD. The substrate for DVD-RAM has athickness of 0.6 mm, and two substrates each having a thickness of 0.6mm are laminated to construct DVD-RAM. In addition, ultra-highrecording-density optical discs comprising a substrate that has adiameter of 120 mm and a thickness of 0.6 mm are being investigated, onwhich information is recorded and reproduced with a blue laser rayhaving a shorter wavelength (about 400 nm).

On the other hand, another trial is being made for increasing thedensity of optical discs of which the thickness is 1.2 mm like that ofconventional CD, etc. For example, rewritable optical discs (CD-R) havebeen proposed, which have grooves having a track pitch width of from 0.3to 0.6 μm and a depth of from 170 to 250 nm, or pits having a width offrom 0.4 to 0.7 μm and a depth of from 280 to 400 nm formed on asubstrate having a thickness of 1.2 mm (see, for example, JapanesePatent Application Laid-Open (JP-A) Hei-9-7232).

For producing substrates for high-density optical discs such as DVD andothers which are being much developed in those days, a 2P method(photopolymerization method) is being investigated, which comprisesforming a UV-curable monomer layer on a transparent substrate such as aplastic substrate or the like, then airtightly applying a stamper havinga reverse pattern for the fine structure of pits (or guide grooves) tothe UV-curable monomer layer, and exposing the UV-curable monomer layerto UV rays via the substrate to thereby polymerize and cure the monomersin the layer to form pits (or guide grooves) on the substrate (forexample, see JP-A Hei-9-106585). According to the 2P method forproducing optical disc substrates, it is possible to make the UV-curablemonomer having a low viscosity reach the deepest site of the finestructure for pits (or guide grooves) as formed on the stamper. In thatmethod, the UV-curable monomer having been spread to the deepest site ofthe fine structure for pits (or guide grooves) is cured, and therefore,it is possible to make the substrates have the fine shape of pits (orguide grooves) transferred thereon with high accuracy. However, the 2Pmethod for producing optical disc substrates is problematic in that themass-producibility therein is lower than that in an injection-moldingmethod for producing substrates, and that the production costs for itare high.

FIG. 4 is a schematic view showing the production of an optical discsubstrate through injection molding, in which a synthetic resin beinginjected via the nozzle tip of a molding machine is charged into acavity (optical disc substrate-shaped space). In the injection-moldingmethod for producing optical disc substrates, a stamper 4 having areverse pattern for the fine structure of pits (or guide grooves) formedon its surface is disposed in a mold 3 to give the cavity 7. Inproducing optical disc substrate in the method, the mold 3 is controlledat a predetermined temperature and clamped, and a synthetic resin 6having been injected via the nozzle tip of a molding machine is chargedinto the cavity 7. After the synthetic resin 6 has reached the deepestend of the cavity 7 (this corresponds to the outer edge of the substratebeing formed), it is compressed into the fine structure for pits orothers of the stamper. Next, this is kept as such for a predeterminedperiod of time to thereby cool and solidify the entire resin in thecavity including the resin to be the center of the substrate, wherebythe fine structure of the stamper is transferred onto the solidifiedresin. Next, the clamped mold 3 is opened, and the optical discsubstrate formed Is taken out of the mold 3. In the injection-moldingmethod for producing optical disc substrates, when the resin having beencharged into the cavity is contacted with the wall of the mold (the wallforms the cavity), the heat of the resin is transferred to the cavitywall immediately after the contact. With the decrease in the resintemperature, the viscosity of the resin increases. As a result of thetransference of the resin heat to the cavity wall, formed is a cooledand solidified layer 5. With the growth of the layer 5, the cavity 7 isfilled with the resin. The problem of the reduction in the patterntransferability due to the formation of the cooled and solidified layerin the injection-molding method noted above is seen also ininjection-compression molding for producing high-density optical discsubstrates.

The injection-molding method has the advantages of goodmass-producibility and low production costs. However, the cooled andsolidified layer formed in the method brings about the problem ofretarding the pattern transferability and increasing the birefringenceof the substrate formed, whereby the quality of the substrate islowered. Where high-density optical disc substrates having finer pits(or guide grooves) than those of conventional ones are produced in theinjection-molding method, the problems to be caused by the formation ofthe cooled and solidified layer will be more serious.

Specifically, with the increase in the density of the pattern for pits(or guide grooves) formed on the surface of the stamper to be used, theresin could hardly enter the depth of the hollows of the fine structureof the pattern thereby often causing transfer failure. For example, evenwhen substrates for DVD-RAM, which are for recording on both the insideof the guide grooves (hollows) and the top between the adjacent guidegrooves (hills) through phase conversion, are intended to be formedunder the conventional injection-molding conditions, the resin couldhardly enter the space to be between the adjacent guide grooves andtherefore good hills could not be formed on the substrates. Even if thesubstrates with such no good fills thereon are used to produce opticaldiscs, good recording and reproduction on the discs produced isimpossible and the discs are failed products.

In order solve this problem, a method for producing thin substrates foroptical discs through injection-compression molding has been developed,in which the temperature of the mold is set higher than that inconventional methods and the clamping force for the mold is switchedduring molding operation (see, for example, JP-A Hei-7-176085). Themethod may be effective for preventing the increase in the viscosity ofthe resin being charged into the mold and for reducing the birefringenceof the substrates produced. However, for the method, the substratematerial is limited to only a specific molding resin. In addition, inthis method, since the mold temperature is high, the substrates releasedfrom the mold after having been cooled and solidified are easilydeformed. Moreover, still another problem with the method is that thecooling time in the method must be long and the molding cycle isprolonged.

On the other hand, it is written in “Reports in Optical Memory Symposium'86”, page 173 and the following pages, that (1) a polycarbonate resinis injection-molded into optical disc substrates (CD substrates) havinga diameter of 130 mm and a thickness of 1.2 mm, at a resin filling rateof 79 cm³/sec, and (2) a polymethyl methacrylate resin isinjection-molded into optical disc substrates (CD substrates) having thesame diameter and thickness as above, at a resin filling rate of 61cm³/sec. In “Polymer Reports”, Vol. 49, No. 8, page 703 and thefollowing pages, a reference is made to the production of CD substrates.Briefly, they say therein that the increase in the resin filling rate inproducing CD substrates is effective for improving the patterntransferability onto the substrates. However, there is found noliterature that suggests the relationship between the patterntransferability and the resin filling rate in injection-molding forproducing high-density optical disc substrates, such as DVD substrateswhich are more small-sized and thinner than conventional optical discsubstrates. Further the technologies described in above two literatureare related to disc substrates (CD substrates) having a thickness of 1.2mm. In case of injection molding of such thick substrates at a resinfilling rate of about 60 cm³/sec, resin filling time is required about 2seconds, so the resin temperature decreases and the viscosity of theresin increases while the resin is filled up. The increase of theviscosity of the resin causes the lowered pattern transferability.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the problemsnoted above, and its object is to provide a method for producing opticaldisc substrates having a lowered degree of birefringence, in which thepattern transferability onto the substrates produced is high and themass-producibility of the substrates is also high.

In order to attain the object, the invention provides a method forproducing optical disc substrates having a diameter of from 80 to 120 mmand a thickness of from 0.5 to 0.7 mm, through injection molding orinjection-compression molding, which is characterized in that a resinfor the substrates is injected and charged into the cavity of a mold ata resin filling rate of not lower than 65 cm³/sec, said resin fillingrate being obtained by dividing the cavity volume (cm³) of the mold,into which the resin is charged, by the time (sec) taken from the startof resin injection through the tip of the nozzle of theinjection-molding machine to the arrival of the resin at the deepest endof the cavity of the mold. Preferably, the resin filling rate is notlower than 80 cm³/sec. The resin filling rate of not lower than 65cm³/sec is roughly converted into the resin filling time of not upperthan 0.1 seconds. The resin filling rate of not lower than 80 cm³/ secis roughly converted into the resin filling time of not upper than 0.08seconds.

The resin filling rate is so settled that the melt viscosity of theresin is within the range of from 1 to 30 Pa·s, within the period oftime of from the start of resin injection through the tip of the nozzleof the injection-molding machine to the arrival of the resin at thedeepest end of the cavity of the mold. Also preferably, the resinfilling rate is so settled that the temperature of the resin having beencharged into the cavity to be around the inner surface of the mold ishigher than the flow-stopping point of the resin, at which the resin ofbeing in a gum-like flat range changes to be within a transition range,within the period of time of from the start of resin injection throughthe tip of the nozzle of the injection-molding machine to the arrival ofthe resin at the deepest end of the cavity of the mold. The resin aroundthe inner surface of the mold as referred to herein is meant to indicatethe surface part of the resin being in the mold cavity and having adepth of not larger than 10 μm from the inner surface of the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the temperature-dependent change in thevisco-elasticity of PMMA.

FIG. 2 is a graph showing the simulation data of the time-dependentchange in the temperature around the surface of a molded product ofPMMA.

FIG. 3 is a schematic perspective view showing the cross-sectionalstructure of pits or guide grooves as formed on the surface of anoptical disc substrate.

FIG. 4 is a schematic cross-sectional view showing the production of anoptical disc substrate through injection molding, in which a syntheticresin being injected via the nozzle tip of a molding machine is chargedinto the cavity of the mold.

In FIGS. 3 and 4, 1 is a guide groove, 2 is the top between the adjacentguide grooves, 3 is a mold, 4 is a stamper, 5 is a cooled and solidifiedlayer, 6 is a resin melt, and 7 is a cavity.

DETAILED DESCRIPTION OF THE INVENTION

PMMA is used as the material for optical disc substrates. Itstemperature-dependent elasticity was measured, and the data obtainedwere plotted to give the curve shown in FIG. 1. With the decrease in theresin temperature, the resin, PMMA of being In a gum-like flat rangechanged to be in a vitreous condition via the transition region. Thetemperature at which the resin of being in a gum-like flat range changesto be within a transition region Is referred to a s a flow-stoppingpoint of the resin. As in FIG. 1, the flow-stopping point of PMMA is128° C. The resin of which the temperature is not higher than itsflow-stopping point does not flow. Therefore, where substrates foroptical discs are formed from a resin in an injection molding method orthe like, fine pits (or guide grooves) could not be transferred onto thesubstrates when the temperature of the resin is lower than theflow-stopping point thereof. In the method, immediately after thefilling of the resin into the mold, the viscosity of the resin havingbeen filled varies in different sites in the direction of the thicknessof the optical disc substrate to be formed, or that is, the viscosity ofthe resin in the region nearer to the inner surface of the mold ishigher. While the stamper pattern is transferred onto the resinsubstrate by compression, the resin is cooled and solidified whilereceiving high shear force, resulting in that the residual stress in theresin being solidified is increased. As a result, the degree ofbirefringence of the cooled and solidified resin substrate shall belarge.

In FIG. 2, plotted are the data of PMMA, which is the same as that usedin obtaining the data in FIG. 1. In this, the data were obtained throughcooling analysis simulation by computer, and show the relationshipbetween the time (seconds) from the start of resin injection forinjection molding or injection-compression molding and the temperature(° C.) of the resin around the inner surface of the mold (the surfacepart of the resin being in the mold cavity and having a depth of 10 μmfrom the inner surface of the mold). The simulation made herein was fornon-steady heat conduction analysis using MARC, in which the temperatureof the resin being injected was 280° C., and the temperature of the mold(this was made of carbon steel having a heat conductivity of 1060×10⁻⁵cal/mm/sec/° C.) was 85° C. As in FIG. 2, it is known that thetemperature of the resin at the depth of not larger than 10 μm from theinner surface of the mold, which has an influence on the fine patterntransferability onto the surface of the resin substrate, becomes lowerthan the flow-stopping point (128° C.) of the resin in about 0.1 secondsafter the resin has begun to be in contact with the inner surface of themold. Where the resin is formed into optical disc substrates accordingto the injection molding or injection-compression molding method underthe same condition as that for the simulation noted above, the viscosityof the resin having been charged into the mold cavity shall be greatlyincreased if the resin filling rate is so low that the introduction ofthe resin to the deepest end of the cavity takes about 0.1 seconds orlonger In that case, the flow-stopping region of the resin will appearon a part of the pattern-transferred surface of the optical discsubstrate before the resin could reach all the deepest sites of the finestructure formed on the stamper (that is, before the resin could reachthe bottom of each hollow of the pattern formed on the stamper, in whichthe hollows correspond to the hills of the resin substrate to beformed). As a result, even if the resin is pressed against the stamperin order to transfer the fine pattern of the stamper onto the resinsurface, transfer failure is inevitable. In addition, in that case, thedegree of birefringence of the optical disc substrate formed shall beenlarged.

In the method for producing optical disc substrates of the invention, itis desirable that the resin filling rate is settled so high that thetemperature of the resin having been charged into the cavity to bearound the inner surface of the mold is higher than the flow-stoppingpoint of the resin, at which the resin of being in a gum-like flat rangechanges to be within a transition range, during the injection chargingof the resin into the mold cavity, or that is, within the period of timeof from the start of resin injection through the tip of the nozzle ofthe injection-molding machine to the arrival of the resin at the deepestend of the cavity of the mold. Specifically, in the method of theinvention for producing optical disc substrates having a diameter offrom 80 to 120 mm and a thickness of from 0.5 to 0.7 mm, the resinfilling rate is a high rate of not lower than 65 cm³/sec, preferably notlower than 80 cm³/sec. Accordingly, in the method, the resin having beencharged into the cavity can reach the deepest end of the cavity whileits temperature is not still lower than the flow-stopping point of theresin, resulting in that the viscosity of the resin being pressedagainst the stamper to enter the depth of each hollow of the finestructure pattern for pits (or guide grooves) formed on the surface ofthe stamper is kept low.

In the present invention, the melt viscosity of the resin is preferablywithin the range of from 1 to 30 Pa·s, within the period of time of fromthe start of resin injection through the tip of the nozzle of theinjection-molding machine to the arrival of the resin at the deepest endof the cavity of the mold. When the resin is PMMA, the melt viscosity ofPMMA is preferably within the range of from 3 to 6 Pa·s. When the resinis PC, the melt viscosity of PC is preferably within the range of from 5to 15 Pa·s. The melt viscosity as referred to herein is meant to thecalculated value from the relationship between shearing velocity andviscosity within the period of time of from the start of resin injectionthrough the tip of the nozzle of the injection-molding machine to thearrival of the resin at the deepest end of the cavity of the mold. Theshearing velocity is calculated from the shape of flow path of the moldand the flow velocity of the melted resin. The relationship between theshearing velocity and the viscosity at molding temperature is measuredby the fixed speed system capillary rheometer for the small hole flowtest method, which is widely used. Further the melt viscosity of theresin within the period of time of from the start of resin injectionthrough the tip of the nozzle of the injection-molding machine to thearrival of the resin at the deepest end of the cavity of the mold can beanalyzed by the resin flow analyzing simulation system of GeneralElectric (GE) Co., which is one of the resin flow computer analyzingsimulation system for designing the mold for injection molding of athermoplastic resin.

The substrate produced according to the method of the invention may belaminated with a reflective thin film, a recording thin film and others,depending on its use, to form a single-sided optical disc. Twosubstrates each laminated with a reflective thin film, a recording thinfilm and others may be combined together to form a double-sided opticaldisc.

In the invention, employable is any resin transparent to the wavelengthsof laser rays for information recording and reproduction and capable ofbeing molded through injection molding or injection-compression molding.Concretely, the resin includes, for example, acrylic resins such aspolymethyl methacrylate (PMMA), etc., as well as polycarbonate (PC)resins, polyolefin resins, polystyrene resins, polyester resins, etc.

Now, the invention is described in more detail with reference to thefollowing Examples and Comparative Examples, which, however, are notintended to restrict the scope of the invention. In those Examples andComparative Examples, used was an injection-molding machine of Fanac'sRoboshot α-50B. Precisely, a discoid stamper was fitted to the mold, inwhich were molded optical disc substrates having a diameter of 120 mmand a thickness of 0.6 mm through injection-compression molding. Thestamper has a reverse pattern for guide grooves having a track pitch of1.0 μm, a groove width of 0.5 μm and a depth of 150 nm, within theradial range of from 23 mm to 58 mm. As the molding material, used wasan acrylic resin (Kuraray's Parapet H-1000SD).

On the surface of each substrate thus molded, formed was a platinum filmhaving a thickness of about 10 nm through sputtering, and thecross-sectional profile of the guide grooves formed thereon was measuredwith a scanning tunnel microscope (STM). For this, two points werechecked, one being near the inner circumference (at a radius of about 24mm) and the other being near the outer circumference (at a radius ofabout 57 mm). From the data of the cross-sectional profile thusmeasured, obtained was the groove depth. The thus-obtained groove depthwas divided by the groove depth of the stamper at the same position, andthe percentage of the resulting value was obtained. This is hereinafterreferred to as “degree of pattern transference”. In addition usingMizoziri Optical's Ellipsometer DVD-36L, the birefringence distributionin the radial direction of each substrate molded herein was measured.The timing for switching the primary clamping force to the secondaryclamping force in the process of Examples and Comparative Examples wasimmediately after gate cutting of the spool part at the end of thepressure retention.

The molding conditions for Examples 1 to 6 and Comparative Examples 1 to3 are shown in Table 1 below. The results of the tests for evaluatingthe properties of the substrates obtained in Examples 1 to 6 andComparative Examples 1 to 3 are shown in Table 2 below. In the mold usedin Examples, the resin filling is finished within 0.1 seconds when theresin filling rate is 65 cm³/sec, or within 0.08 seconds when the resinfilling rate is 80 cm³/sec. In Table 1, the column for the clampingpressure indicates the pressure in the filling and pressing step, thatin the initial cooling step, that in the middle cooling step, and thatin the last cooling step, in that order. The melt viscosity of the resinfor Examples 1 to 5, in which cylinder temperature is each not lowerthan 285° C. and mold temperature are not lower than 80° C., is each notlower than 3 Pa·s within the period of time of from the start of resininjection through the tip of the nozzle of the injection-molding machineto the arrival of the resin at the deepest end of the cavity of themold. The melt viscosity of the resin for Example 6, in which cylindertemperature is 280° C. and mold temperature is 80° C., is not lower than5 Pa·s.

TABLE 1 Resin Cylinder Mold Resin Filling Temper- Temper- FillingClamping Rate ature ature Time Pessure (cm³/sec) (° C.) (° C.) (sec)(ton) Example 1 65 285 85 0.10 17 → 27 → 24 → 0 Example 2 65 290 87 0.1017 → 27 → 24 → 0 Example 3 80 285 80 0.08 22 → 32 → 27 → 0 Example 4 80285 85 0.08 22 → 32 → 27 → 0 Example 5 80 290 87 0.08 22 → 27 → 24 → 0Example 6 80 280 80 0.08 17 → 27 → 24 → 0 Comparative 55 290 87 0.12 22→ 32 → Example 1 27 → 0 Comparative 48 290 87 0.14 22 → 32 → Example 227 → 0 Comparative 35 280 80 0.18 22 → 27 → Example 3 24 → 0

TABLE 2 Degree of Pattern Transference (%) Birefringence (nm) innerouter inner center outer circum- circum- circum- circum- circum- ferenceference ference ference ference Example 1 100 95 15 10 5 Example 2 10095 15 10 5 Example 3 100 100 10 10 5 Example 4 100 100 10 10 5 Example 5100 100 10 10 5 Example 6 95 90 15 10 5 Comparative 100 70 12 10 0Example 1 Comparative 100 60 13 10 0 Example 2 Comparative 80 50 20 15 5Example 3

As is obvious in Table 2, the thin, optical disc substrates obtained inExamples of the invention all have improved pattern transferability, ascompared with those obtained in Comparative Examples, and each has adegree of pattern transference of 100% throughout from its innercircumference to its outer circumference. Further optical discsubstrates obtained in Examples 1 to 5, in which melt viscosity of PMMAis each within the range of from 3 to 5 Pa·s, each has a degree ofpattern transference of 100% throughout from its inner circumference toits outer circumference.

As has been mentioned in detail hereinabove, injection molding orinjection-compression molding with good mass-producibility is employedin the method of the invention, in which are produced good substratesfor optical discs without lowering the pattern transferability thereoneven when stampers with higher density for pits (or guide grooves) areused than those for conventional CD.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A method for producing optical disc substrateshaving a diameter of from 80 to 120 mm and a thickness of from 0.5 to0.7 mm, comprising charging and injection molding orinjection-compression molding an acrylic resin for the substrates intothe cavity of a mold at a resin filling time of not greater than 0.08seconds, wherein a melt viscosity of the resin is within the range offrom 3 to 6 Pa·s, within the period of time of from the start of a resininjection through the tip of the nozzle of the injection-molding machineto the arrival of the resin at the deepest end of the cavity of themold, wherein the temperature of the resin charged into the cavitynear/around the inner surface of the cavity is higher than theflow-stopping point of the resin, the point at which the resin being ina gum flat range changes to be within a transition range, within theperiod of time of from the start of resin injection through the tip ofthe nozzle of the injection-molding machine to the arrival of the resinat the deepest end of the cavity.
 2. A method for producing optical discsubstrates having a diameter of from 80 to 120 mm and a thickness offrom 0.5 to 0.7 mm, comprising charging and injection molding orinjection-compression molding an acrylic resin for the substrates intothe cavity of a mold at a resin filling rate of not lower than 65cm³/sec, said resin filling rate being obtained by dividing the cavityvolume (cm³) of the mold, into which the resin is charged, by the time(sec) taken from the start of resin injection through the tip of thenozzle of the injection-molding machine to the arrival of the resin atthe deepest end of the cavity of the mold, wherein a melt viscosity ofthe resin is within the range of from 3 to 6 Pa·s, within the period oftime of from the start of resin injection through the tip of the nozzleof the injection-molding machine to the arrival of the resin at thedeepest end of the cavity of the mold.
 3. The method of claim 2, whereinthe resin filling rate is not lower than 80 cm³/sec.
 4. A method forproducing optical disc substrates having a diameter of from 80 to 120 mmand a thickness of from 0.5 to 0.7 mm, comprising charging and injectionmolding or injection-compression molding an acrylic resin for thesubstrates into the cavity of a mold at a resin filling time of notgreater than 0.1 seconds, wherein a melt viscosity of the resin iswithin the range of from 3 to 6 Pa·s, within the period of time of fromthe start of resin injection through the tip of the nozzle of theinjection-molding machine to the arrival of the resin at the deepest endof the cavity of the mold.
 5. The method of claim 4, wherein the resinfilling time is not greater than 0.08 seconds.